The present invention relates generally to a technique for regulating a DC source voltage, and more particularly to a method and circuit configuration for regulating a DC source voltage when the output voltage required is less than the DC source voltage.
Various circuits have been devised to regulate battery voltage or other DC source voltages in situations in which the output voltage required is greater than the source voltage. Some of these conventional circuits, even address the need for a well-regulated output voltage. For example, one known method of regulating battery voltage is that of placing an isolated power supply, of reasonable efficiency, in series with the battery voltage, wherein only a small portion of the battery power is processed. If a voltage error correcting circuit is designed to sense the entire output voltage, tight regulation can be maintained over varying battery voltage and load conditions.
In addition, various circuits have been devised to regulate battery voltage in situations in which the required output voltage is less than the source voltage and galvanic isolation is not required. Often times, an efficient method is required that yields maximum efficiency when the input to output differential voltage is relatively small.
Turning now to the circuit of FIG. 1(a) and the corresponding graph of FIG. 1(b), the basic simplified circuit operation can be described. First, at time T=0, a bucking capacitor C1 is fully discharged, placed in series with the DC source, and the output load Rload is at the DC source voltage (in this example, 27 volts). As illustrated in FIGS. 1(a) and 1(b), the capacitor C1 voltage will increase over time as it accumulates charge, causing a corresponding reduction in the output voltage across Rload. In this example, as shown in FIG. 1(b), the output voltage will ultimately fall to zero.
If however, a method were implemented to remove the capacitor C1 charge, in a controlled and continuous manner, the charge accumulating in C1, and consequently the voltage across C1, could be maintained at any desired level. Since the output voltage is equal to Vsourcexe2x88x92V(C1), the output voltage could also be maintained at any desired level, simply by varying the voltage across C1. In addition, if the charge could be recycled (i.e., returned to the source), the circuit efficiency will be vastly improved.
One known technique in which to remove or extract the charge across capacitor C1 is illustrated in FIG. 2. As shown, a switch S1, in series with a resistor R1, is placed across capacitor C1. By closing S1, the charge across capacitor C1 is dissipated through resistor R1, thereby reducing the C1 charge buildup. However those skilled in the art will certainly appreciate that this technique has the disadvantage of being lossy (i.e., of xe2x80x9closing powerxe2x80x9d).
Accordingly, it is desired to develop a technique for regulating a DC source voltage in a manner to provide an output voltage that is less than the source voltage while maintaining maximum efficiency. More specifically, it is desired to develop a technique for removing the capacitor C1 charge, in a controlled and continuous manner, thereby providing the ability to maintain the output voltage at any desired level by varying the voltage across C1, while also recycling the removed charge (i.e., returning the removed charge to the source), thereby vastly improving the overall circuit efficiency.
The present invention solves these problems by providing an efficient method and circuit configuration for regulating DC source voltage.
The method, and corresponding circuit, for extracting charge from a capacitor to maintain a fixed voltage across an output load, the voltage across the output load being smaller than a source voltage, includes the steps of sensing an output voltage that indicates a voltage drop across the capacitor, comparing the sensed output voltage to a voltage reference source and varying the on-time of a switch connected across the capacitor, the switch connected in series with a transformer, based on the result of the comparing step, thereby controlling the voltage across the capacitor and the output voltage.
In one embodiment of the invention, the sensing step is performed by a voltage divider circuit and the comparing step is performed by a voltage error correcting amplifier.
In a further aspect of the invention, when the switch turns on, the method includes the steps of connecting the capacitor across the primary winding of the transformer, thereby superimposing the voltage charge on the capacitor across the secondary winding of the transformer and creating an alternating transformer secondary current, converting the alternating transformer secondary current to half-cycle DC current and filtering the half-cycle DC current into steady state DC current.
Again, in a specific embodiment of the invention, the converting step is performed by a rectifier having an anode connected to a first terminal of the secondary winding and the filtering step is performed by an inductor and a capacitor.
In yet a further aspect of the invention, when the switch turns off, the method further provides for recovering and recycling the energy stored in the transformer and in one embodiment, uses a clamp winding and rectifier to perform this function.
In a specific embodiment of a circuit for extracting charge from a first capacitor to maintain a fixed voltage across an output load, the circuit includes a first capacitor connected in series between the DC source voltage and the output load, a transformer and a switch connected in series with the transformer, the series combination being connected across the first capacitor. A pulse width modulator generates drive pulses to drive the switch, a high side driver is coupled between the switch and pulse width modulator, a voltage divider circuit senses an output voltage and an error correction amplifier, coupled between the voltage divider circuit and pulse width modulator, compares the output voltage sensed by the voltage divider circuit to a voltage reference source. The circuit operates such that the pulse width modulator generates drive pulses to vary the on-time of the switch based on an output signal from the error correction amplifier and modifies the on-time of the switch to maintain a fixed voltage across the output load.
In a yet further aspect of the invention, the circuit includes a first rectifier having a first side coupled to a first terminal of the secondary winding of the transformer and converting the transformer secondary current to half-cycle DC current, a second capacitor having a second side coupled to a second terminal of the secondary winding of the transformer and a second rectifier having a first side coupled to the second terminal of the secondary winding of the transformer and a second side coupled to a second side of the first rectifier. The second rectifier allows the transformer secondary current to flow continuously during the time intervals when the switch is off. An inductor is coupled between the second side of the second rectifier and a first side of the second capacitor, and, together with the second capacitor, filters the half-cycle DC current from the first rectifier into steady state DC. An output filter capacitor is coupled between a second side of the first capacitor and ground and a third rectifier has a first side coupled to a second side of the inductor. A clamp winding is coupled between a second side of the third rectifier and ground, and the clamp winding and the third rectifier recover and recycle the magnetizing energy stored in the transformer during xe2x80x9coff-timexe2x80x9d of the switch. Specifically, when the switch closes, it connects the first capacitor across the primary winding of the transformer, superimposing the voltage across the first capacitor across the secondary winding of the transformer, and the transformer allows the charge removed from the first capacitor to be fed back to the DC source voltage.